HIPS resins are well known in the synthetic organic polymer art as a class of thermoplastics which offers excellent mechanical properties as well as good chemical resistance. The general characteristics of HIPS resins are described, for example, in "Modern Plastics Encyclopedia," McGraw-Hill, New York, N.Y., 1990, p 96. HIPS resins are "rubber-modified" copolymers or homopolymers of styrenic compounds. They are known for their ease of processing, good dimensional stability, impact strength, and rigidity. They generally are prepared by dissolving an elastomer in a styrenic monomer before polymerization of the latter, but the elastomer can also be incorporated in other ways.
For purposes of the instant invention, a HIPS resin is a thermoplastic which can be a homopolymer or random or block copolymer produced by polymerizing one or more vinyl aromatic monomers such as styrene, alkyl-substituted styrenes, halo-substituted styrenes, or alpha-alkyl styrenes or substituted styrenes, in the presence of an elastomer monomer or polymer, such as polybutadiene, for example. Further, a formulated HIPS resin, for all purposes herein, comprises at least about 50 wt % HIPS resin. The remainder of the formulated HIPS resin comprises various monomeric or polymeric additives which modify the properties of the HIPS resin. These additives include, for example, various impact modifiers, stabilizers, processing aids, pigments, flame retardants, synergists, etc. Such additional additives can be incorporated into the HIPS resin in various ways.
Incorporation of the various additives is not a trivial matter, and the properties of articles made from the formulated HIPS resin can be affected by the manner in which it is done. When the additives are solids, dry-blending can be employed. For example, the solids can be mixed and heated to soften and homogenize the mass, which can then be sheeted, chopped, and pelletized.
A flame retardant, such as a halogenated organic compound, is often incorporated into a formulated HIPS resin so as to constitute as much as about 20-25 wt % of the formulation. Such incorporation can adversely affect the properties, other than the flammability, of the formulated HIPS resin. For example, a flame retardant having a high melting point range, such as decabromodiphenylethane, or decabromodiphenyl oxide will have a filler type effect on the HIPS resin formulation, whereas a flame retardant having a lower melting point range will have a plasticizer effect on the formulation. Because of the effect flame retardants have on articles containing such HIPS resin formulations, improvements in such formulations and formulation techniques are highly desirable and of significant commercial importance.